No, it's not chemtrails. Instead, it's a type of cloud called a Fallstreak Hole. You may recognize that it glimmers in a way that's similar to the contrails of jets. That's because a similar phenomenon causes it. That rainbow look is the result of water crystals in the clouds freezing and refracting the light. Those frozen crystals have also made one patch of the cloud slightly heavier, pulling it downward from the rest of the cloud layer, and giving it that odd, punch-out look.

I mentioned contrails earlier because they are also caused by frozen water droplets hanging in the air, gleaming in the sunlight. But contrails happen when warm air is released with jet exhaust and collides with supercooled water in the stratosphere. This causes the water to freeze instantly, and thus you can see a long trail of frozen vapor where a plane just flew overhead.